In many applications such as audio applications, it may be useful, in terms of maximizing the output power, to use one or several pairs of amplifiers to drive the load in a bridge mode. FIG. 1 shows a typical single-input bridge configuration of a pair of single input operational amplifiers A and B. In the example shown, the noninverting input (+) of one of the two amplifiers is connected to ground, the B amplifier in the example, and the inverting inputs (-) of the two operational amplifiers A and B are connected with a resistor R2.
In order to maximize the output power, it is important to fully exploit the output voltage swing of both amplifiers of FIG. 1, when there are asymmetric characteristics of saturation of the two amplifiers A and B. By observing the scheme of FIG. 1, the gain of the amplifier A is:
Va=Vin*(1+R1/R2)
whereas the gain for the other amplifier B is: EQU Vb=-(R1/R2).
The B amplifier is not directly driven by the input signal, but, by the voltage BV1 present on the inverting input node (-) of the A amplifier. In these conditions, when the A amplifier reaches saturation, the voltage V1 remains at the reached value and the B amplifier may not follow the other to a state of saturation. This Phenomenon is illustrated in FIG. 2.
A commonly used approach is to make the gains of the two amplifiers substantially equal, eventually by using different values of resistance of the feedback resistors, R1, as shown for example in FIG. 3. If the following equation is satisfied, a simultaneous saturation of the two outputs Va and Vb will be ensured: EQU Va=1+R1a/R2=Vb=1R1b/R21.
However, owing to residual asymmetries of the resistors, and above all of the saturation characteristics of the two amplifiers A and B which may be different in terms of respectively high and low saturation characteristics, it is difficult to ensure an identical saturation of both.